1. Field of the Invention
The present invention relates to a pair of measuring electrodes, to a biosensor comprising at least one pair of measuring electrodes of this type, to an electrochemical cell comprising a biosensor of this type and to a process for producing the pair of measuring electrodes.
2. Related Prior Art
Numerous pairs of measuring electrodes and biosensors of this type are known from the prior art; cf. for example WO 99/07879, WO 00/62047 or WO 00/62048, as well as the very wide range of possible applications and uses described therein.
The biosensors described in these documents have arrays of pairs of measuring electrodes which can be addressed individually and each of which pairs has two electrodes which are arranged parallel to one another in one plane and may be designed as fingers entangled into one another, so-called interdigitated electrodes, or as interleaved, concentric sections of a circle.
The known biosensors are used, for example, to determine concentrations of biomolecules, to determine physico-chemical properties, to detect immune reactions and the like. In general terms, biosensors of this type are used in analytical or immunological assays, where they are used as amperometric sensors to measure extremely low concentrations. However, the measuring electrodes can also be used for electrostimulation, for electrophoretic enrichment or separation of charged molecules or, for example, for electrochemical recording of reaction sequences.
The amperometric sensors mentioned detect currents which emanate from oxidation or reduction reactions at molecules in solution in the vicinity of the electrodes. A selectivity for a specific molecule species can be achieved because certain redox-active molecules are reduced or oxidized at a specific potential. The current measured is proportional to the concentration of the molecules in the solution.
Depending on the type of reaction involved, the transferred charge per molecule is one or a few elemental charges, whereby it is possible to increase the sensitivity by what is known as redox recycling. For this purpose, the electrodes are arranged at a very short distance from one another, so that a redox-active molecule can diffuse to and fro between the anode for the oxidation reaction and the cathode for the reduction reaction with a high probability. In the process, the molecule takes up charge a number of times at the cathode (reduction) and releases it again at the other electrode, the anode (oxidation); cf. Niwa et al., Electroanalysis 3 (1991), 163-168.
To enable the redox recycling phenomenon to be exploited, the dimensions of and distance between the electrodes must be as small as possible in order to allow a rapid diffusion of the molecules between anode and cathode. Niwa et al., loc. cit., in this context describe two different electrode arrangements, in which the distances between anode and cathode and the widths of anode and cathode are as little as 1 μm and the length is 2 mm. In one embodiment, up to 100 interdigitated fingers lie next to one another in one plane, while in the other embodiment anode and cathode fingers are vertically spaced apart by an insulation, resulting in a regularly structured array having microstrips. The electrodes are produced by conventional photolithography and etching techniques.
In order to achieve the selectivity, which is not sufficient in the case of the vertical arrangement, the authors propose a further reduction in the dimensions or the use of selected polymers.